MacCHESS, a synchrotron radiation research resource for macromolecular crystallography, has the overall goal of advancing the frontiers of structural biology through innovate technical research and development. MacCHESS builds upon the National Science Foundation investments in the Cornell High Energy Synchrotron Source (CHESS), which maintains the synchrotron radiation laboratory, and the Laboratory for Nuclear Studies, which operates the storage ring. At MacCHESS, technical developments are driven by both core and collaborative research projects involving a broad range of macromolecules. In addition to performing technical R&D and core research, MacCHESS provides specialized instrumentation for macromolecular crystallography and a staff for user training and equipment maintenance. MacCHESS has established itself as one of the most productive facilities in the world for macromolecular crystallography with over 350 papers published during the past five years as a result of MacCHESS related activities. Of these, 65 were published in the high visibility journals, Science, Nature and Cell. Currently, CHESS stations A-1 and F-1 are used primarily for monochromatic oscillation data collection and station F-2 is used for MAD phasing experiments. A key component of this proposal is the development of two new stations for macromolecular crystallography, G-1 and G-2. Developing these stations for crystallography will be cost effective because funding for G-line will be provided by the NSF with matching funds for Cornell University. During the next five years, MacCHESS technical R&D will focus on (1) new x-ray optics, (2) CCD and pixel array x-ray detectors, (3) new instrumentation for small crystals, crystallography and phasing, (4) new software for data collection, analysis and transfer, (5) innovate uses of the world wide web to create a macromolecular crystallography "collaboratory", (6) upgrading of computing hardware and (7) instrumenting G-line. Core and collaborative research will focus on (1) novel phasing methods, (2) implementation of x- ray detectors, (3) applications in molecular medicine, (4) large macromolecular structures and (5) G-line commissioning. MacCHESS will continue to provide service in a wide variety of scientific areas. Particularly notable is the past record of productivity in the area of AIDS-related structural biology. Technical developments resulting from MacCHESS research are freely available to the scientific community and will be disseminated through meetings, workshops and other mechanisms. We will work towards increasing interactions with other synchrotron sources in an effort to maximize the benefits of our efforts.